/Engine/Source/Runtime/MeshDescription/Public/MeshAttributeArray.h
Set an attribute value for the given element ID. Note: it is generally preferable to get aTMeshAttributesRefand set multiple elements through that.
template<typename T> void SetAttribute ( const ElementIDType ElementID, const [FName](API\Runtime\Core\UObject\FName) AttributeName, const int32 AttributeChannel, const T & AttributeValue )